Note: On models equipped with Traction Control (Traction Control), ABS performs two functions: anti-lock braking system, as well as the functions of the automatic stability control and traction control system (ASC+T).
1. ABS is installed on most models as basic equipment, on all other models it can be installed on request. The system contains a hydraulic unit, which includes electromagnetically driven hydraulic valves, an electric return pump, four wheel sensors (one for each wheel) and an electronic control unit (ECU). The system is designed to prevent wheel locking during sudden braking. This is achieved by automatically reducing the braking force on the corresponding wheel and then increasing the force.
2. The electromagnetic valves receive a control signal from the ECU, which in turn receives signals from four wheel sensors that monitor the rotation speed of each wheel. By comparing these signals, the ECU determines the vehicle's speed. The ECU uses this speed value to determine when one of the wheels is slowing down too slowly compared to the vehicle's speed, and thus determines which wheel is at risk of locking. In normal mode, the system operates in the same way as a braking system without ABS.
3. When the ECU determines that one of the wheels is in danger of locking, it sends a signal to the corresponding valve of the hydraulic unit. This valve cuts off the brake caliper of the wheel that is in danger of locking from the master cylinder, blocking the pressure supply to it from the hydraulic system.
4. If the wheel speed continues to decrease to an excessively low value. The ECU switches on the return pump, which pumps the brake fluid back into the master cylinder. Due to this, the pressure in the brake caliper decreases so much that the wheel is released. As soon as the wheel speed increases again to an acceptable value, the pump stops and the hydraulic valve opens, connecting the brake caliper to the master cylinder. As a result, the brake is applied again. This cycle can be repeated up to 10 times per second.
5. The operation of the valves and the return pump creates pulsations in the hydraulic system circuits. When the ABS is operating, such pulsations can be felt on the brake pedal.
6. The operation of the ABS is entirely dependent on the transmission of electrical signals. To prevent the system from being triggered by abnormal signals, a safety unit is built into it, which monitors all signals received by the ECU. When an abnormal signal is received or when a decrease in battery voltage is detected, the ABS is automatically switched off, and the ABS malfunction indicator lights up on the instrument panel. In this case, normal braking is performed without the participation of the ABS.
7. If an ABS malfunction occurs, the vehicle must be taken to the dealer for diagnostics and repair.
8. In models equipped with the ASC+T system, a hydraulic accumulator is added to the hydraulic system. The operation of the ABS system is similar to that described above, and in addition, the ASC+T function is added to it. If, during acceleration of the car, one of the wheels accelerates to an abnormally high speed, the corresponding brake is applied, preventing slippage of this wheel. If a malfunction occurs in the ASC+T system, you must contact a BMW dealer.
[The original article is located on: BMWman.ru]